Warner et al. used more sophisticated equipment for a study of the half-red shrimp
Sergestes similis. A PMT was positioned below an animal harnessed in a spherical
aquarium that could be illuminated from above with a variable intensity of blue light
(520 nm). Irradiance could be varied from 0 to 3 × 10−4 μW cm−2. Bioluminescence
levels were measured with the phototube in the few milliseconds after the
conditioning light was shut off. The result (Fig. 12.3) shows a nearly exact match
between the dorsal conditioning intensity and ventral luminescence output. In this
animal, the organ of Pesta, located below the gut, produces the light. The gut is darkly
pigmented and is the only part of the body that would be silhouetted against the
downwelling light for a predator below. Warner et al. extended the experiment by
comparing the matches for shrimp with their eyes covered with transparent masks and
opaque masks. With opaque masks the organ of Pesta produced no light at all. The
eyes must measure the downwelling light to allow the match.
Fig. 12.3 Comparison of light output of the organ of Pesta in Sergestes similis to the
conditioning irradiance above, showing the close match.
(^) (Simplified from Warner et al. 1979.)